The Function and Connection Method of Potentiometer

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Introduction

A potentiometer is actually a variable resistor. Because it functions in the circuit to obtain an output voltage that is related to the input voltage (applied voltage), it is called a potentiometer.

Article Core

Potentiometer

Purpose

Introduce what the function of potentiometer and the connection method of potentiometer are.

Application

Semiconductor industry.

Keywords

Potentiometer

Catalog

Introduction

Potentiometer Overview

Circuit Graphic Symbol

Main Parameters of the Potentiometer

Resistance Change Characteristics of Potentiometer

Potentiometer Resolution

Potentiometer's Maximum Operating Voltage

Potentiometer Noise

Potentiometer Structure and Type

Potentiometer Application

(1) Dimming Table Lamp Circuit

(2) DC Stabilized Power Supply Circuit

Potentiometer Connection

Power Amplifier General Fault Repair

1.The whole Machine Doesn’t Work

2. No Sound Output

3.Sound Light

4.Large Noise

5.Distortion

6.Howling

7.Howling

(1). Power and Grounding Point Processing

(2). Output Stage

(3). Preamplification Section

Potentiometer Overview

Potentiometer

Circuit Graphic Symbol

The unit of the potentiometer resistance is the same as the resistor, and the basic unit is also ohm, which is represented by the symbol Ω. The potentiometer is represented in the circuit by the letter R or RP (old standard W), and Figure 1 is its circuit graphic symbol.

2. The resistance value marked on the potentiometer is called the nominal resistance.

3. The rated power of the potentiometer refers to the maximum power that can be consumed by the long-term continuous load at the specified rated temperature in the DC or AC circuit when the atmospheric pressure is 87~107kPa. The rated power series for wirewound and non-wirewound potentiometers are shown in Table 1.

Table 1 Nominal series of potentiometer rated power (unit: power)

Potentiometer

0.025, 0.05, 0.25, 1, 1.6, 2, 3, 5, 10, 16, 25, 40, 63, 100

Wire wound potentiometer

0.25, 0.5, 1, 1.6, 2, 3, 5, 10, 16, 25, 40, 63, 100

Non-wire wound potentiometer

0.025, 0.05, 0.1, 0.25, 0.5, 1, 2, 3

Resistance Change Characteristics of Potentiometer

The resistance change characteristic refers to the relationship between the resistance value of the potentiometer and the change of the length of the movable contact or the rotation angle of the rotating shaft, that is, the resistance output function characteristic. There are three commonly used resistance change characteristics, as shown in the figure.

Figure 2 potentiometer resistance curve

Straight type (X type): As the position of the moving point changes, the change in resistance is close to a straight line.

Exponential (Z type): The change in the resistance of the potentiometer is exponentially related to the change in the position of the moving corner point.

（1）The linear resistance of the linear potentiometer changes linearly with the angle of rotation. When the conductive material on the resistor body is evenly distributed, the resistance per unit length is approximately equal. It is suitable for applications where uniform adjustment is required (eg voltage divider).

（2）The exponential potentiometer is unevenly distributed due to the conductive material on the resistor body. When the potentiometer starts to rotate, the resistance value changes slowly. When the rotation angle increases, the resistance value changes steeply. The exponential potentiometer allows the power to be unequal in unit area, and the end of the resistance change is allowed to withstand a large amount of power. It is commonly used in volume adjustment circuits because the human ear is most sensitive to the sound of the sound. When the volume is loud to a certain extent, the hearing of the human ear becomes dull. Therefore, the volume adjustment generally uses an exponential potentiometer to make the sound change appear smooth and comfortable.

（3）The logarithmic potentiometer has a non-uniform distribution of the conductive material on the resistor body. When the potentiometer starts to rotate, its resistance value changes rapidly. When the rotation angle increases, when it rotates to the end near the resistance value, the resistance value The change is slow. Logarithmic potentiometers are suitable for use in electronic circuits that are contrary to the requirements of exponential potentiometers, such as contrast control circuits and tone control circuits for televisions.

Potentiometer Resolution

The resolution of the potentiometer is also called the resolution. For the wirewound potentiometer, when the moving contact moves one turn, the output voltage changes discontinuously. The ratio of this change to the output voltage is the resolution. The theoretical resolution of a linear wirewound potentiometer is the reciprocal of the total number of turns of the winding N and is expressed as a percentage. The higher the total number of turns of the potentiometer, the higher the resolution.

Potentiometer's Maximum Operating Voltage

The maximum working voltage of the potentiometer refers to the highest operating voltage that the potentiometer can work under long-term reliable operation under the specified conditions, which is also called the rated working voltage.

The actual operating voltage of the potentiometer is less than the rated operating voltage. If the actual working voltage is higher than the rated working voltage, the potentiometer's power exceeds the rated power, which will cause the potentiometer to overheat and damage.

Potentiometer Noise

When the potentiometer slides on the resistor under the action of applied voltage, the electric noise generated is called the dynamic noise of the potentiometer. Dynamic noise is one of the main parameters of sliding noise. The magnitude of dynamic noise is related to the speed of rotating shaft, the contact resistance between contact point and resistor body, the non-uniform change of resistivity of resistor body, the number of dynamic contact points and the magnitude of applied voltage.

Potentiometer Structure and Type

The potentiometer consists of a housing, a sliding shaft, a resistor body and three terminals, as shown. There are many types of potentiometers. According to the adjustment method, they can be divided into rotary (or rotary handle) and straight slide potentiometers. According to the number of joints, they can be divided into single-connected and multi-connected potentiometers. There are two kinds of switches and switches; according to the resistance output function characteristics, it can be divided into three types: linear potentiometer, exponential potentiometer and logarithmic potentiometer. Such as solid potentiometers, chip potentiometers, carbon film potentiometers, glass glaze potentiometers, precision conductive plastic potentiometers and other potentiometers.

Potentiometer Application

(1) Dimming Table Lamp Circuit

Figure 4 shows a simple and practical dimming table lamp circuit. Adjusting the resistance of RP can change the charging time of capacitor C to reach the UG worth time, that is, adjust the conduction angle of the thyristor, so that the thyristor triggers conduction earlier or later, thereby adjusting the output voltage of the thyristor so that the voltage across the lamp can be 0. Change between ~220V. The voltage is high, the light is bright; the voltage is low, and the light is dark.

Figure 3 ordinary potentiometer structure diagram

Figure 4 dimming table lamp circuit

(2) DC Stabilized Power Supply Circuit

The DC stabilized power supply circuit is shown in Figure 5. Generally, R4 can select low-power carbon film potentiometer and RP to select high-power wire-wound sliding potentiometer. Adjusting the resistance of R4 can change the level of the output voltage U. Adjusting the RP can test the load capacity of the power supply.

Potentiometer Connection

The general potentiometer, the middle is the moving piece, so when measuring the resistance, the pin 3 is the total resistance, the moving piece will not change the moving value, and the resistance will increase from the clockwise direction (moving the piece) 2, 3 is the opposite. 6 feet are called double potentiometers, that is, 2 single units are together, 8 feet should be with a switch, generally used in car audio.

Figure 7 potentiometer connection

Why is the audio-grade playback device exaggerated by the volume adjustment buttons?

It has recently been discovered that all devices that are used as artifacts have one thing in common, and the volume adjustment buttons are very large and exaggerated. From what point of view is this design considered?

The volume adjustment knob is actually a potentiometer behind it. Its principle is as shown in Figure 8:

Figure 8

The black part is a resistive film. Gray is the contact point, which will produce different resistance values when changing, thus changing the potential (volume).

The way it is made into components is:

The above is mono, three pins.

Below is the double potentiometer, stereo, 6 pins:

There are also non-rotating, but linear sliding, the principle is the same as the rotating.

On the mixer, and the volume fader for the colorful C4, this is the case:

Most potentiometers use a carbon film to make a resistive film. After using it for a period of time, it will cause noise, which is caused by the wear and tear of the toner. Then you need to buy a can of WD40 to lubricate. I suggest that when the new equipment is in the door, use the WD40 to go through the maintenance before you start using it.

If the manufacturing process is better, the durability will increase a lot.

In this regard, it is generally believed that the quality of the carbon film acoustic potentiometer of Japan Alps is a good drop.

Common Alps RK27/RK09/RK08 potentiometers

Alps RK27 for desktop equipment

Alps RK08 for handheld devices

Because these potentiometers all use a blue outer casing, they are often referred to as "Alps blue shell potentiometers".

Alps has another top-grade carbon film potentiometer RK40/50 series, which is specially designed for advanced audio:

The audio equipment that can be used with the RK40/50 has undoubtedly entered the ranks of middle and high-level Hi-Fi.

But what potentiometer is used for Hi-End top equipment? - is a step potentiometer.

Instead of sliding on a single piece of resistive film, it divides the resistance value into several levels. Each stage is soldered with an independent resistor (the resistance values used for the left and right channels are strictly matched), and then the silver contacts are used to connect step by step. The resistance of each level is fixed.

For example, I bought the Shanghai Weixin knife holder in Taobao + the stepping potentiometer of the American Dale military resistance resistance welding (the following two photos were taken by myself on the desk):

After the potentiometer is placed on the black cylindrical aluminum alloy casing, it is installed in the chassis, you can see that it is much larger than the knob outside the casing:

The stepper potentiometer will never cause noise due to the film peeling off, nor will the left and right channel volume be inconsistent due to wear and aging, so it is the most reliable mechanical potentiometer for audio equipment.

Seeing now, you know: those audio equipment that make the volume knob very large, if it is not because the potentiometer inside is really big, it is to use a large knob to imply that the potentiometer used in it is advanced. Goods to achieve the purpose of raising the price.

Finally, put a few pictures of the well-recognized brand stepper potentiometer for everyone to see:

In addition to the large and very high end of the potentiometer itself mentioned above, the knobs are large enough to play the real role of these advanced potentiometers. Imagine that the diameter of the knob is too small. When the knob is rotated, a slight movement will cause a large change in the potential. Only if the knob radius is large enough, the potentiometer can be finely adjusted, and this is not limited to the volume. Adjustment.

Just the right resistance torque, plus a knob of sufficient size, these high-end potentiometers can satisfy some discerning ears.

The potentiometer can be understood as a voltage divider, then this reference-level voltage divider should be the favorite of people, the uncertainty is 0.1ppm, and the most subtle volume adjustment can be achieved.

As shown below:

Schematic diagram:

Power Amplifier General Fault Repair

Common faults of HI-FI audio and AV amplifiers are that the whole machine does not work, no sound output, light sound, loud noise, distortion, howling.

The following describes the troubleshooting ideas and troubleshooting techniques for various faults.

1.The whole Machine Doesn’tWork

The failure of the whole machine does not work. After the power is turned on, the amplifier has no display, and all the function keys are invalid, and there is no sound, just like when the power is not supplied.

The power circuit should be checked first during maintenance. Use a multimeter to measure the DC resistance at both ends of the power plug (the power switch should be turned on). Normally, there should be hundreds of ohms. If the measured resistance is much smaller and the power transformer is severely heated, it indicates that the primary circuit of the power transformer has a partial short circuit; if the measured resistance is infinite, check whether the fuse is blown, whether the primary winding of the transformer is open, the power cord and the plug There is no disconnection between them. Some machines have added temperature protection devices, and the temperature fuses are connected in the primary circuit of the power transformer (usually installed inside the power transformer, the insulation paper outside the transformer can be removed), and it will also make the power transformer primary after damage. The circuit is open.

If the resistance of both ends of the power plug is normal, you can measure whether the output voltage of the power supply circuit is normal. For amplifiers that use a system-controlled microprocessor or logic control circuit, it is important to check that the supply voltage (usually +5V) of the control circuit is normal.

If there is no +5V voltage, the input voltage of the three-terminal regulator IC 7805 should be measured. If the input voltage is not normal, the rectifier and filter circuits should be checked. If the voltage of the 7805 input terminal is normal, and the output terminal has no voltage of 10V or the voltage is low, the load can be disconnected to see if the +5V voltage can return to normal. If the +5V voltage is normal, the fault is in the load circuit; if the +5V voltage is still abnormal, the fault is in the 7805 itself.

If the +5V supply voltage of the system control circuit is normal, check whether the clock and reset signal of the microprocessor are normal, whether the key control and display drive circuit are damaged.

2. No Sound Output

The silent fault is manifested when the function keys are operated, and there is a corresponding status display, but no signal is output.

When repairing an amplifier with a protection circuit, it should be seen whether the protection relay can be closed after power on. If the relay does not operate, measure whether the output voltage of the power amplifier circuit is offset or whether the overcurrent detection voltage is normal. If the midpoint output voltage is offset or the overcurrent detection voltage is abnormal, it indicates that the power amplifier circuit is faulty. Check whether the positive and negative power supplies are normal. If the positive and negative voltages are asymmetrical, the load circuit of the positive and negative power supplies can be disconnected to determine whether the power supply circuit itself is abnormal or the power amplifier circuit is faulty. If the positive and negative power supplies are normal, check the amplifier tubes in the power amplifier circuit for damage.

If the point output voltage and the overcurrent detection voltage in the power amplifier circuit are normal, and the protection relay does not pick up, the fault is in the protection circuit. The relay drive integrated circuit or the drive tube should be checked for damage and the detection circuits are normal. If the relay contacts can be pulled in, but there is no sound output, first check if the speaker is normal, the relay contacts are in good contact, and the squelch circuit is operating.

If the above parts are normal, then use the signal interference method to check whether the fault is in the power stage or the front stage circuit. Use the R×1 block of the multimeter to ground the red test lead. The black test lead quickly touches the input end of the post-amplifier circuit. If there is a strong “sweet” sound in the speaker, the fault is in the preamplifier circuit; if the speaker is not available In response, the fault is in the post-amplifier circuit.

For integrated circuit power amplifier circuits that do not use peripheral protection circuits (usually with thermal protection inside the integrated circuit), the power supply voltage can be measured first or not. If the power supply voltage is normal, check with the signal interference method: add DC intermittent signal to the signal input end of the power amplifier integrated circuit. If the speaker has a strong "tick" sound, the power amplifier integrated circuit is normal, and the fault is in the preamplifier circuit. If there is no "tick" sound, and the relevant peripheral components are also normal, the fault is in the power amplifier integrated circuit itself.

The tube amp has no sound output, and should also check its power supply to see if the filament is bright and the temperature of the tube is normal. If the filament is not bright, the tube is very cold. Check whether the filament and the screen voltage of the power amplifier tube are normal or not. If the voltage is not normal, go back and check the power circuit. If necessary, disconnect the power load circuit to determine if the power circuit is faulty or the load is shorted. If the voltage is normal, a DC intermittent interference signal can be added to the center of the volume potentiometer. If there is a strong reaction, the post-stage amplification circuit is normal, and the fault is in the pre-amplifier circuit; otherwise, the fault is in the post-stage amplification circuit. Interference signals can be added to the gate of the push tube and the gate of the input amplifier tube respectively, and the stage-level plus interference signal does not respond, indicating that the circuit behind the stage is not working properly. Suspicious components (such as electron tubes) can be overhauled by substitution.

An AV amplifier with Dolby Surround decoding function, if the Doppy surround sound state is silent and the main channel sound is normal in the through state, in the case of a normal power supply circuit, it is usually a Dolby Surround decoding circuit. Or the system control circuit is not working properly. If the channels are silent in both surround and pass-through modes, check the system control circuitry, signal selection circuitry, and total volume control circuitry.

3.Sound Light

The so-called sound light fault refers to the audio signal in the process of amplification and transmission, because the amplification amount of a certain amplification stage changes or is attenuated at a certain point, so that the gain of the amplifier decreases or the output power becomes smaller.

When overhauling, first check if the signal source and speaker are normal, and you can use the replacement method to check. Then check the various types of transfer switches and control potentiometers to see if the volume can get bigger.

If all the above parts are normal, it should be judged whether the fault is in the previous stage or in the latter stage. For a certain channel sound, the signal output of the front stage circuit can be input to the subsequent stage circuit of the other channel. If the sound size of the speaker is unchanged, the fault is in the latter stage circuit; otherwise, the fault is in the previous stage. Circuit.

The sound caused by the post-amplifier circuit is light, mainly due to two reasons: insufficient output power and insufficient gain. It is possible to determine which cause is caused by appropriately increasing the input signal (for example, adding the signal output from the recorder to the speaker directly to the input of the rear stage power amplifier circuit, changing the volume of the recorder, and observing the change in the output of the power amplifier). If the input signal is increased, the output sound is large enough, indicating that the power output of the power amplifier is sufficient, but the gain is reduced. It should be checked whether the contact resistance of the relay contact increases, the input coupling capacitance decreases, and the resistance of the isolation resistor increases. Negative feedback capacitor capacity becomes smaller or open circuit, negative feedback resistor resistance increases or opens. If the input signal is increased, the output sound will be distorted and the volume will not increase significantly, indicating that the output power of the post-amplifier is insufficient. First check whether the positive and negative supply voltages of the amplifier are low (if only one channel is light) , it is not necessary to check the power supply), whether the performance of the power tube or integrated circuit is deteriorated, and whether the resistance of the emitter resistor is increased or not.

The sound caused by the switch and potentiometer in the pre-stage circuit is light and easy to find by visual inspection, which can be cleaned or replaced. If it is suspected that a signal coupling capacitor fails, it can be tested in parallel with the same value capacitor; the performance of the amplifier tube or op amp integrated circuit is poor, and it can also be checked by the substitution method. In addition, there is a problem with the negative feedback component, which also causes the circuit gain to drop.

4.Large Noise

The noise of the amplifier is hum, pop, induced noise and white noise.

During maintenance, you should first determine whether the noise is from the previous stage or from the latter stage. The signal connection plugs of the front and rear stages can be removed. If the noise is significantly smaller, the fault is in the front stage circuit; otherwise, the fault is in the latter stage circuit.

AC sound refers to the low-pitched, monotonous and stable 100Hz AC hum, which is mainly caused by poor filtering of the power supply. It should be checked whether the power rectifier, filter and voltage regulator components are damaged. The decoupling capacitors at the power supply terminals of the front and rear stage amplifier circuits are either weakly soldered or failed, and a low frequency oscillation noise similar to the hum is also generated.

Inductive noise is a complex and harsh humming component, mainly due to poor switching of the front-end circuit, poor grounding of the potentiometer, or poor signal wiring shielding.

The popping sound refers to the intermittent "劈啪" and "咔咔" sounds. In the pre-stage circuit, it should be checked whether the signal input plug and the socket, the transfer switch, the potentiometer, etc. are in poor contact, and the coupling capacitor has no solder joint or leakage. The post-amplifier circuit should check whether the relay contacts are oxidized, whether the input coupling capacitor is leaking or has poor contact. In addition, the soft input breakdown of the differential input tube or constant current tube in the latter stage circuit also produces "咔咔" noise similar to electric spark.

White noise refers to the random continuous “sand” sound, which is usually caused by the poor performance of the input stage transistor, FET or op amp integrated circuit in the front and rear stage amplifying circuits. The component of the specification is replaced by a test.

5.Distortion

The distortion fault is caused by the offset of the working point of a certain amplification stage or the work of the output stage of the power amplifier. During the inspection, the specific fault location can be judged according to the change of the output power and distortion of the amplifier.

If the tube amplifier is distorted and the output power becomes small (sound is light), it should be checked whether the aging of an amplifier tube in the push-pull amplifier, the working point is incorrect, or the output transformer is partially short-circuited, causing its work imbalance; if the distortion is at the same time, the output power becomes larger. Mostly due to the resistance change in the negative feedback circuit, the capacitor failure or the bypass capacitor short circuit of the cathode self-generated bias.

If the distortion of the transistor amplifier increases significantly with the increase of the volume, check whether the operating point of a transistor of the push stage is offset (usually in a power amplifier without a protection circuit) or the capacitance distortion in the feedback circuit; If the size is distorted, the fault is in the preamplifier circuit, and the operating point of each amplifier tube should be checked for offset.

Abnormal operation voltage of the integrated circuit amplifier or internal damage of the power amplifier integrated circuit can also cause distortion (referring to a machine without a protection circuit).

6.Howling

The howling fault is caused by self-excitation in the circuit, and is divided into low-frequency howling and high-frequency howling.

Low-frequency whistling refers to the low-frequency "beep” sound, usually due to poor power supply filtering or decoupling (often accompanied by humming while whistling), should check the power supply filter capacitor Whether the regulator and the decoupling capacitor are open or fail, so that the internal resistance of the power supply increases. Poor performance of the power amplifier IC, there will also be low frequency howling faults, at this time the operating temperature of the integrated circuit will be very high.

The frequency of high frequency whistling is relatively high, which is usually caused by the failure of the high frequency absorbing capacitor in the amplifying circuit or the deterioration of the performance of the preamplifier integrated circuit. It can be checked by connecting a small capacitor to both ends of the damping capacitor or decoupling capacitor of the post-amplifier circuit. In addition, when the negative feedback element is damaged, changed or desoldered, high frequency positive feedback will also occur and high frequency howling will occur.

7.Reduce Noise

Some cheap amps are screaming when they are turned on, which not only affects the sound quality, but also makes people upset. Here are a few ways to deal with it:

(1). Power and Grounding Point Processing

Many amplifier filter capacitors are small, four are only about l000μF, and have a CB capacitor of 0.22μF at both ends, which can not only reduce the hum of the power amplifier, but also improve the transient of the power amplifier. Strength and high frequency resolution. Some power amplifiers may still have a grounding point even after the above processing, and the grounding point may be improper. Generally, the grounding point should be selected near the filter capacitor, and the "one-point grounding method" should be used.

(2). Output Stage

If the static potential of the output stage deviates from the zero point, it will produce a great humming sound. At this time, it may be a zeroing resistance or a problem with the output tube. The output point potential of the power amplifier can be carefully adjusted, which should be below 100mv. At zero point, you should carefully check whether some components of the power amplifier, such as the tube, are damaged.

(3). Preamplification Section

First, short the input end of the preamplifier to ground to see if the noise disappears. If the noise disappears, the noise can be determined to be from the input signal line. It can be replaced with a three-core shielded cable. Note that the shield can only be grounded at one end. In addition, the coupling capacitor should use a small leakage capacitor, such as tantalum capacitor, MKP capacitor, etc., the volume potentiometer housing should be grounded.